This invention relates to a push-pull type amplifier circuit and, more particularly, to an aplifier circuit with a stabilized bias in its output stage.
In an output stage of a conventional power amplifier, idling current is generally subjected to a substantial change due to factors including irregularity in characteristics of elements and, accordingly, adjustment of idling current is required. When, for example, a transistor in an output stage has been replaced, readjustment of idling current to compensate for change in bias resistance is required. Besides, compensation due to temperature change by thermal coupling is indispensable for prevention of thermal runaway and a special circuit design for this purpose is required.
For achieving this purpose, a technique, for example, been proposed (e.g., Japanese Patent Publication No. Sho 54-12031, FIGS. 3 and 4) to detect voltage across emitter resistance of an output transistor and control base bias of the output transistor on the basis of the detected voltage.
However, resistance inserted in the emitter of the push-pull type output transistor is one way for achieving operational balance and hence is of an extremely small value of 1 ohm or less. Accordingly, change in detected voltage due to the emitter resistance is very small in the case of a small current operation, though it may not be so in the case of a large amplitude operation, and this makes it difficult to maintain accuracy in the bias control. Increase in the emitter resistance for increasing detected voltage, on the other hand, poses the problems of reduction in the drive capacity and increase in output impedance of the amplifier.
In sum, in a conventional circuit design of an output stage of a power amplifier, adjustment of idling current and temperature compensation by thermal coupling are indispensable and a special circuit design is required for achieving this purpose. In the method for carrying out the bias control on the basis of voltage across the emitter resistance of the output transistor, an accurate control is difficult. Increase in emitter resistance for realizing an accurate control results in deterioration of amplifier characteristics such as increase in the output impedance.
It is, therefore, an object of the invention to provide a push-pull type amplifier circuit capable of obviating adjustment of idling current without bringing about deterioration in the amplifier characteristics and without using a large type diode for bypassing purpose.